An NPC "moonwalks"

As part of my newly started education in gamedevelopment I encoutered a peculiar issue: we have each progammed an NPC so when the player came within a certain distance of the NPC, the NPC would react by moving towards the player. However, in my project, the NPC "moonwalks" away from the player, a development which have left my teacher stumped.

Here is the code in question, the programming language is C#:

 using UnityEngine;
 using System.Collections;
 
 public class Troll : MonoBehaviour {
 
     Animation anim;
     GameObject player;
 
     // Use this for initialization
     void Start () {
         anim = GetComponent<Animation> ();
         anim.Play ("Idle_02");
         player = GameObject.FindGameObjectWithTag ("Player");
     }
     
     // Update is called once per frame
     void Update (){
         
         if (Vector3.Distance (transform.position, player.transform.position) < 3) {
                         Vector3 dir = new Vector3 (player.transform.position.x, transform.position.y, player.transform.position.z);
                         transform.LookAt (dir);
                         anim.Play ("Attack_01");
                         rigidbody.velocity = new Vector3 (0, rigidbody.velocity.y, 0);
                 } else if (Vector3.Distance (transform.position, player.transform.position) < 20) {
                         Vector3 dir = new Vector3 (player.transform.position.x, transform.position.y, player.transform.position.z);
                         transform.LookAt (dir);
                         anim.Play ("Run");
                         rigidbody.AddForce (transform.forward * 100);
                 } else {
                         anim.Play ("Idle_01");
                 }
         }
     }
 

An odd detail is that if I change "rigidbody.AddForce (transform.forward * 100);" to a negative number, the NPC will move forward. The NPC is the "Earthborn Troll" from the asset store, in case that is an important detail.

I would greatly appreciate any help.